Cells respond to extracellular cues by activating intracellular signaling pathways that transmit that signal to target proteins, thereby enabling the cell to mount an appropriate physiological response. In a number of cases, signal transduction pathways that operate in the same cell share components, raising questions as to how specificity of signaling is maintained. This issue also obtains in the yeast Saccharomyces cerevisiae. We will use genetic and molecular methods to learn about the mechanisms that confer specificity to signal transduction pathways in yeast. Because many proteins that function in signaling in yeast have counterparts in other organisms, including humans, we expect that mechanisms we discover will apply in other species too. Activation of signal transduction pathways often leads to a change in the transcription program for the responding cell, but signaling pathways also influence facets of cell biology other than transcription. Our second goal is to identify and understand the connections that link the pheromone, filamentous growth, and osmosensing pathways to machinery that controls function of the actin cytoskeleton and progression through the cell cycle. Ste20, a p21-activated protein kinase that has human homologs, is one point of connection to the actin cytoskeleton. A second point of connection is a poorly studied ubiquitin-like system that we discovered is required for filamentous growth. Based on the phenotype of mutants lacking this system, we hypothesize that it influences regulation of the cell cycle. We will use genetic and mass spectrometry approaches to identify targets of Ste20 and of the ubiquitin-like system, and investigate the role of those targets in filamentous growth, actin cytoskeleton function, and cell cycle progression. [unreadable] [unreadable]